This invention relates to a method and apparatus for measuring the permeability and for estimating the thickness of refractory coatings which have been applied to a porous substrate such as a foundry sand mold or core.
In preparing a foundry mold or core to produce a cast metal part, refractory coatings may be applied to the surface of the mold or core so that the molten metal is presented with a high quality contact surface. A properly applied coating of the correct thickness and permeability prevents defects such as penetration of the molten metal into the mold, spalling of the mold into the metal, and the passage of gas in either direction. The result is a greater yield of castings free of defects such as non-metallic inclusions, pinholes, pocks and so forth.
These refractory coatings consist essentially of a suspension of finely divided materials such as silica or zircon in water or isopropyl alcohol (or other suitable carriers), but may also contain certain well-known additives to improve rheological properties and shelf life. The coating is applied to the mold or core by painting, spraying, dipping, or by any number of available methods. Once in place, the coating is dried by the application or generation of heat or by simply allowing the liquid portion to evaporate at room temperature. The application and drying procedure is repeated as many times as necessary to build up the desired thickness of refractory coating.
The ability of the refractory coating to improve casting quality has been found to depend on many factors; chief among these are the thickness of the coating and its permeability. If the refractory coating is too thin, there is the possibility of metal penetration; if the coating is too thick, there may be "wash crazing" and spalling. On the other hand, permeability of the coating layer exerts direct control on the movement of gas at the mold-metal interface and, therefore, on certain gas related defects such as blows and pocks. For example, when permeability is too low, gas can become trapped in a sector of the mold causing incomplete fill defects.
Heretofore, instruments and devices known to measure these two characteristics of the coating layer on a foundry mold or core have been deficient in some significant respect. For example, instruments easily movable to make measurements from place to place on a large mold or core have not been available. Portability is of particular importance in certain foundry operations in which very large castings are made. For example, it is not uncommon in producing parts for a large steam turbine for foundry personnel to actually move from place to place inside a large mold in an attempt to gather permeability and refractory coating thickness data. Still further, in obtaining mold or core permeability data with many of the prior art devices the mold or core may be rendered useless (but damaged in any case) after testing since the mold or core of interest must be damaged or destroyed to accommodate the test apparatus. In this regard, see U.S. Pat. Nos. 2,659,433; 3,172,258; 3,181,346 which are of potential interest for permeability testing but which require destructive sampling of the mold or core. It is clear, too, that the apparatus of these patents, as well as that of U.S. Pat. No. 3,335,787, (which is a complicated automatic compactor-tester) have no potential use for measuring the permeability of a refractory coating in place on a mold or core.
In other fields, not directly related to foundry processes, devices have been devised for testing the gas permeability of such materials as paper and fabric. For instance, in U.S. Pat. No. 3,808,876 a permeability tester is disclosed for testing the permeability of a surface material such as paper or cloth. While seemingly portable, the permeability tester of U.S. Pat. No. 3,808,876 requires a source of compressed air (not shown in the patent specification) which limits its portability. Further, use of such a device is not as straightforward as would be desired for foundry use since there is a necessity for finely adjusting a vacuum-box or needle-valve to achieve essentially uniform measuring pressure as determined by a vacuum or pressure gage. Furthermore, the required use of precision fitted parts, such as the needle-valve, contribute to long-term unreliability when operating in the inherently dirty, dusty environment of a foundry.
In U.S. Pat. No. 4,191,046 there is disclosed a permeability tester directed principally to permeability measurements on cigarette papers. In this device, the paper under test is mounted in a paper clamping head and air pressure is adjusted to a standard test pressure while the air flow rate is noted. Thus, many of the drawbacks of the above-noted patent U.S. Pat. No. 3,808,876 are present in this latter disclosed paper testing apparatus.
It is, therefore, among the objects of the present invention to provide a simple, easily operated method and apparatus for determining the thickness and gas permeability of refractory coatings as such coatings are in place on foundry molds and cores. Still further, it is an object of the invention to provide apparatus for carrying out such determinations which is entirely portable, requires no connections to utility supplies such as electricity, air sources and so on, and which is hand-held during operation. still further objects of the invention will become apparent from the ensuing description.
As sometimes used herein, the generic term "mold" will be intended to encompass both mold and cores--the latter being recognized as that part of a mold which forms the interior of a hollow casting.